This invention relates to an image sensor and, in particular, to an autofocus (automatic focus) image sensor of a correlation detection system.
Autofocusing of a correlation detection system uses a special image sensor to realize its function. In the special image sensor, it is required to obtain a uniform output signal in a wide range of brightness from a dark condition at night to a bright condition under sunlight. For this purpose, the special image sensor generally incorporates means for maintaining a constant output signal even in a situation where brightness variation occurs.
In order to detect a varying brightness, use is made of average light amount detecting pixels generally called a monitoring sensor (or monitoring pixels) to control an exposure time (accumulation time) so that electric charges (signals) generated in a pixel array are kept constant. The correlation detection system is a system for detecting a degree of focusing from locations of two images separated by a separator lens. The image sensor compliant with the correlation detection system is provided with two pixel arrays, called a standard portion and a reference portion, respectively, linearly arranged and adapted to read a same image. Since the same image is read, no functional problem is generally caused as long as the monitoring sensor is disposed in the vicinity of one of the pixel arrays.
An example of a conventional structure of the above-mentioned image sensor is shown in FIGS. 1 and 2. FIG. 1 shows a schematic diagram of the image sensor and FIG. 2 shows a block diagram of a signal control portion thereof. The autofocus image sensor comprises a standard portion 7, a reference portion 8, and a signal control portion 6. The standard portion 7 comprises a CCD (charge transferring portion) 1-1, a pixel array charge detector 2-1, a monitoring charge detector 2-3, a pixel array 3-1, average light amount detecting pixels (hereinbelow referred to as monitoring pixels) 4-1, and a control electrode 5-1. The monitoring pixels 4-1 is for use in monitoring an average light amount of the pixel array 3-1. The pixel array charge detector 2-1 is a signal charge detector for detecting signal charges from the pixel array 3-1. The monitoring charge detector 2-3 is a signal charge detector for detecting signal charges from the monitoring pixels 4-1. The control electrode 5-1 controls supply of charges from the pixel array 3-1 to the CCD 1-1. As a result, the control electrode 5-1 controls supply of charges from the pixel array 3-1 to the pixel array charge detector 2-1 connected to an output of the CCD 1-1. The reference portion 8 comprises a CCD 1-2, a pixel array charge detector 2-2, a pixel array 3-2, and a control electrode 5-2. The pixel array charge detector 2-2 is a signal charge detector for detecting signal charges from the pixel array 3-2. The control electrode 5-2 controls supply of charges from the pixel array 3-2 to the CCD 1-2. As a result, the control electrode 5-2 controls supply of charges from the pixel array 3-2 to the pixel array charge detector 2-2 connected to an output of the CCD 1-2. The signal control portion 6 includes a reference voltage generator 13′, a comparator 12′, and an accumulation control signal generating portion 14′.
The signal control portion 6 is controlled by a control signal and the comparator 12′ compares an output from the monitoring charge detector 2-3 of the standard portion 7 with a reference voltage from the reference voltage generator. On the basis of a result of the comparison, the accumulation control signal generating portion 14′ controls the control electrode 5-1 of the standard portion 7 and the control electrode 5-2 of the reference portion 8. However, in order to perform focusing with higher accuracy, it is necessary to control a signal amount into an optimal condition such that a signal (signal charges) of each of the pixel arrays is not saturated. For this purpose, an optimal autofocus image sensor is desired.
Such autofocus image sensor is disclosed in the following patent documents. In Japanese Unexamined Patent Application Publication (JP-A) No. 2005-300844 (corresponding to US2007/098389A1), a standard portion sensor and a reference portion sensor are neighboring in parallel and respective sensor arrays are disposed so as to be relatively deviated from each other. By using the two sensor arrays, signals for focus detection are produced. In Japanese Unexamined Patent Application Publication (JP-A) No. S62-199060, at least two photosensors are provided in the vicinity of a photodiode array. The at least two photosensors produce signals from which a contrast of a subject is detected. Further, Japanese Unexamined Patent Application Publication (JP-A) No. 2001-33687 (corresponding to U.S. Pat. No. 6,445,883B1) discloses a method of reducing focus detection errors by providing a standard portion and a reference portion. Japanese Unexamined Patent Application Publication (JP-A) No. H9-14956 discloses a method of adjusting detection sensitivity.